NO790954L - APPARATUS FOR UNDERWATER MOUNTING OF BEETS - Google Patents

Description

Device for laying pipes in an underwater trench.

The present invention relates to a device which is suitable for pipe laying in an underwater trench.

It is sometimes, e.g. when laying pipelines for oil, water and gas transport, for the installation of electricity and telephone lines under water, it is necessary to lay pipes under water, especially on the seabed.

Laying a pipe in a trench means laying and, if possible, covering the pipe with loose material.

According to conventional technology in this area, the laying was carried out in two phases, i.e.:

a) excavation of the trench and

b) laying the pipe.

When laying on the seabed, apart from the usual problems in connection with the two mentioned phases, problems will arise when it comes to keeping the trench clean in the time between the excavation and the pipe laying. The latter problems can be

quite difficult,

In most cases, after digging the trench, it has been necessary to carry out a further round of digging immediately before laying the pipes in order to prepare parts of the trench which may have collapsed or where obstacles may have accumulated. A solution to this problem includes the device described in Italian patent document 913 731 by the same P<p>Pfinner. The patented solution includes a submerged pump combined with a framework that supports the pump and is displaceable along the pipe that has already been laid on the seabed. The framework includes digging shovels adapted for excavating two trenches, parallel and close to the pipe, so that the pipe is dug again as a result of the bottom zone directly under the already laid pipe collapsing (either spontaneously or induced by means of crumbling drawbars).

In special cases, however, problems and disadvantages will arise even with the use of the above-mentioned device: 1) When the pump bodies are of a similar size and/or the cable to be laid in a trench on the seabed has a small diameter, the frame's center of gravity is raised so that the balance becomes uncertain and operational difficulties arise as a result, particularly where the seabed is uneven. 2) In the device according to the above-mentioned Italian patent document, the framework slides along the pipe by means of rubber-coated rollers, which are kept in contact with the pipe. However, it has been shown that greater security is required for contact between the pipe and the framework, particularly when the pipe has a small diameter. 3) With the known device, the excavated material is emptied further away or spread in the vicinity of the framework with the risk of secondary contamination. 4) With the known device, conditions in the water or the sea (e.g. waves, underwater currents, tides, etc.), especially when the trench digging has to be carried out at great depths, can prevent operation during normal pulling. In particular, it sometimes happens that the device assumes an inclined position in relation to the direction of operation. This brings with it clear disadvantages and cannot be easily detected from the towing and steering pontoon. 5) In the case of special seabed conditions, e.g. cemented sandy bottom, the work of the digging shovels for digging the two trenches, parallel to the pipe to be laid in the trench, will not be sufficient, unless more than one digging round is carried out. 6) During the preliminary stage, when the pipe is laid on the seabed, several pipeline rigs are connected together. Such a connection is usually created by placing a form of metal plate around the two ends to be connected and which meet butt-in-butt, into which cement mortar is poured to protect the joint. For several reasons, the sheet metal form remaining around the joint can be deformed and especially opened, so that it forms an obstacle to the advancement of the re-excavation equipment. 7) In some cases, especially with certain seabed shapes, the dredged material that enters the digging buckets to be fed to the pump chambers is highly viscous. This makes feeding the pump chambers difficult.

The main purpose of the present invention is the elimination of all the mentioned disadvantages and problems, both those related to a particular peculiarity of the seabed and/or those related to the known device, while at the same time the basic advantages of the device according to Italian patent 913 731 is maintained.

For this purpose, a device of the type is used which generally comprises a framework which can be placed across the pipe to be buried, digging or dredging devices mounted on the framework and adapted for operation along both sides of the pipe which is already laid out on the seabed, as that two trenches of a certain depth are formed, which run parallel to and close to the pipe, possibly drawbars or ploughs, either fixed or retractable and adapted for pulverizing the seabed zone directly under the pipe and a pump for removing the material dredged up by the digging bodies . According to the invention, the device is characterized in that the framework comprises

- at least two screeds for slip resistance and adjustment of the maximum digging depth, mounted symmetrically in relation to the axis of the pipe, where the said screeds are continuous or interrupted and have

adjustable height in relation to the digging or dredging organ

- pair of front and/or rear arms, articulated with the framework and held in permanent contact with the surface of the tube via

sliding contact means, preferably rollers}

- a first set of crumbling drawbars, mounted on vertical bars and driven by weight arms driven by ram unitsj where the drawbars are movable between a rest position, where they run substantially parallel to and offset to the pipe, and an operating position, where the drawbars are in contact with

the bottom zone immediately below the pipe to be dug in the trench} - digging shovels, which form the aforementioned digging organs and are mounted in pairs symmetrically in relation to the framework and in a number of two or more shovels on each side of the framework, where each shovel has a digging edge on a level that is predetermined and lower than the level of the digging edge of the closest digging bucket in the direction of the framework's pull} - means for adjusting the distance between the two inner sides of the framework, where the adjustment depends on the diameter of the pipe

to be laid in a ditch}

a second set of crumbling drawbars disposed therein

smallest in the front part of the framework and project down, so that they

makes contact with the bottom in front of the dredger blades; - a tank centrally mounted on the framework for the supply of compressed air or ballast water; - a device with cutting discs for cutting algae, mounted on the front part of the framework.

Among other features that characterize the device according to the invention either in combination or separately, the following can be mentioned: a) a probe member assigned to the first set of crumbling pull irons to prevent accidental damage to the pipe; b) the drawbars or plows adapted to crumble the bottom zone under the pipe may take the form of side extensions of the digging buckets, so that the crumbling is combined with the removal of the crumbled material by means of the pump, the drawbars being retractable, when the excavation of the side trenches is completed ; c) the outlet pipe for excavated or dredged material is laid backwards in relation to the framework, in relation to the direction of pull, so that the excavated material is emptied onto the already reburied pipe at a distance that is not less than the distance where the buried pipe has gained its final position in relation to the bottom; d) When operating on a very deep bottom, the suspension cable for the framework is kept loose, so that disturbing phenomena, such as e.g. wave movement can be absorbed while the pulling of the framework is ensured by a cable running parallel to the bottom and passing over a track disc mounted on the anchoring means, while the catch-up speed of the rear windlass is suitably adjusted; e) the digging shovels and/or the above, other sets of crumbling wooden cores are provided with means that create vibration; the connections between the vanes and/or the drawbars and the rest of the framework are appropriately modified so that said vibration is enabled; f) each digging bucket comprises a device which causes the dredged material entering the bucket to be exposed to a stream of water before it passes to the pumping chamber, said water stream's flow rate and pressure being appropriately controlled; g) in front of each set of rollers that allow the device to slide along the pipe to be laid in a trench, or at least in front of the first set of such rollers with a view to the direction of feed of the device, a funnel-like body is arranged, which comprises two valve-shaped bodies , which is connected along a generatrix of the funnel-like body, where the latter tapers towards the roller set, so that the metal plates from the joint are compressed and pressed closer to the tube before the corresponding roller set makes contact with it; h) organs are arranged for recording the variations in the pressure affecting the sides of the framework, so that variations in the position of the framework are registered;

i) the outlet end of the outlet pipe, especially when emptying the already trenched pipe, is appropriately enlarged to promote deposition of the emptied material without it spreading to nearby areas.

The various features and advantages of the present invention will become clear from the following detailed description of a preferred embodiment, with reference to the drawing, where

fig. 1 is an elevation in schematic form of the device according to the invention,

fig. 2 is a side view of a preferred embodiment of the device according to the invention,

fig. 3 is a representation similar to fig. 2 and shows the device in operating condition,

fig. k and 5 are front views of the device and correspond to fig. 1 respectively fig. 2,

fig. 6 is an elevation of the device according to fig. 5"

fig. 7 is a side view of another embodiment of the device in a rest position,

fig. 8, 9 and 10 are schematic and simplified renderings of a particular detail of the device according to fig. 2nd,

fig. 11 and 12 are schematic side views of the device according to the invention in two operating conditions.

In fig. 1 shows a schematic representation of the device according to the invention, which comprises a framework, generally denoted by 10, which is adapted to be placed in the manner described below across the pipe 11 which is to be laid in a trench. Guide arms 13 are rotatably mounted on the framework 10 on the front and back by means of hinges, the axes of which are indicated at 12. The guide arms 13 are held in contact with the surface of the tube 11 by a roller sled 14. The reference number 15 indicates track discs for pull rope 16, in the event that the trenching is to take place in deep water as envisaged in fig. 12.

In fig. 2-7 shows the device for laying pipes in a trench. The device comprises a framework 10, consisting of two side elements 17, e.g. in the form of two I-beams, and two transverse elements 18.

On the frame, which consists of two side elements 17 and the cross elements 18, the pump bodies 19 are mounted. They correspond to a compressed air butt of the type described in Italian patent document 838 495 (US patent document 3 624 933) by the same inventor. Said patent document describes the construction and operation of the pumps.

Mudi^e or digging buckets 20, which are also described in previously known patents by the same inventor, are combined with the pump bodies 19»

The buckets have a dredged edge 21, which contacts the bottom for digging and they are closed in the front part by a grate 22 (fig. 5)" which limits the size of the dredged material entering the inner bucket cavity to then pass through the inlet connection 23 to the associated pump body 19 by opening an inlet valve not shown. Both the pump and the dredger blades are described in the Italian patent documents 838 495 and 950 389 of the same inventor and which indicate further details.

The frame 10 is further completed by vertical bars 24 and a stepped lower frame, which corresponds to the different operating levels for the vanes 20. Such a lower frame comprises pairs of side elements 25" which support the pump bodies and the vanes at the respective heights.

For sliding movement of the frame along the tube, rollers are provided, indicated at 26 and preferably provided with a rubber-coated surface. The rollers have contact with the surface of the tube 11 in a manner illustrated in fig. 5°g 10"

As shown in fig. 4-7>, the transverse members 18 are extended to the sides to allow attachment of the vertical support members 27 for the screed 28.

More specifically, the support members 27 consist of a cylinder 29, e.g. a hydraulic or pneumatic cylinder with a piston 30.

The piston 30 is articulated at its free end with the help of the ring 31 (fig. 1) and the bolt 32 which is supported by the brackets 33 (34 in fig. 7 or 64 in fig. 2, 4, 5 and 6), which extends parallel to the framework and to the tube's 11 axis. A more detailed description follows below.

As shown in fig. 7", the end of the piston 30 is enclosed by a tubular part 35, which is adapted to slide with the piston during the reciprocating movement in the cylinder 29. In this way, a larger diameter cylindrical part 36 of the same piston is produced, until the conditions shown in fig. 5 has been reached.

For the purpose of biasing the piston 30, represented by the rod 37", the tubular part 35 has a slot which allows the complete return of the piston 30.

In connection with the digging buckets, there is arranged a first set of retractable dredging drawbars 39" which are driven for rotation between a rest position, essentially parallel to the axis of the pipe 11, and an operating position, where the bottom zone immediately below the pipe 11 is contacted, when the trenches on the side of and near the pipe itself is excavated by the shovels 20.

For controlling the drawbars 39, cylinders 40 are arranged, with pistons 4l which are driven out of rods 42 with weight arms 43 which are rotatably articulated with them. The weight arms are attached to the rods 44 which support the drawbars 39 (fig»3»6 and 7)»

As previously mentioned, the drawbars 39 are only activated when this is necessary due to the bottom conditions and to prevent damage to the pipe 11. Probes, which are schematically shown at 45 (fig. 4), are preferably arranged. These probes can e.g. be electrically connected to the drawbar drive means, so that the drawbars 39 are brought to rest when the probes come into contact with the pipe 11.

It should also be noted that the crumbling drawbars

39 by means of the control device shown or with other means not shown can be movable not only between the two above-mentioned positions, but also retractable backwards, especially in the case of sudden obstacles against the crumbling operation. In this way, the obstacle is avoided. The above description of the device according to the present invention relates to a first, simplified embodiment, as generally shown in fig. 7» The cutters 34 are, as shown in fig. 7, designed in several lengths which are connected to each other by means of hinges 38, so that they can better follow an uneven bottom. Instead of the hinges 38, curved or universal connectors can be arranged,

The screeds also have another, equally important function: as a result of their height adjustability, regulation of the digging depth and thus of the re-digging depth of the pipe can be achieved.

If we now look specifically at the characteristic features of the invention, a second set of crumbling drawbars 60 comes in front of the shovels 20. The drawbars form a set in front of each digging shovel to penetrate the bottom, which is thus crumbled before the shovel 20 penetrates it.

The drawbars' 60 form™ and inclination are variable, depending on the current requirements, while their function remains unchanged. In a modification not shown, the drawbars 60 which are arranged between two successive vanes can be attached to both vanes and thus form a reinforcement member.

The action of the drawbars 60 can be supplemented with vibration, preferably exerted against the digging shovels and/or the drawbars by means of vibrators (known per se and therefore not shown in more detail), which is generally indicated in the drawing at 6l.

In order to induce an increased vibration effect, a joint, preferably a rubber joint of a known type, is preferably intermediately connected in the flange 62, which connects each vane 20 to the respective pump body 19.

In the upper wall of each vane 20, a hole has been taken out for the passage of the end of a small tube 63 (fig. 3)1 which partially protrudes into the vane. The pipes 63 are preferably connected to a common collecting pipe l6k which is supplied with water with adjustable pressure and adjustable flow rate.

Instead of small pipes 63, an opening of a suitable size can alternatively be arranged in the upper wall of each shovel 20, so that the water can freely enter the inner shovel cavity, where the dredged material is collected during the digging phase of the shovel. It is obvious that with the help of the pipes 63 it is possible to dilute the dredged material from the corresponding shovel with a suitable amount of water. The material can thus easily enter the pump chamber which is connected to the cavity of the vane via the channel 23.

In the embodiment shown in the latter figures, the cutters for the device for laying pipes in a trench take the form of tank-shaped lengths, as indicated at 64. The cutters are either rotatably articulated with each other or independent and adapted to be supplied either with compressed air (when buoyancy is desired) or with a ballast liquid. Also in this case, the mowers are adjustable in terms of height between a rest position (where they are fully lowered) and an operating position (where they are fully or partially raised). If the device according to the invention e.g. is used on a bottom where the minnows could be expected to sink, the advantage of buoyancy is obvious. If the device knows otherwise

o

for example operated on a sloping bottom where digging is carried out across the slope, it is possible to achieve an excellent balance of the framework by regulating the buoyancy and the ballast effect. This can be achieved by supplying the different milkers with compressed air or ballast fluid. Similar considerations and technical effects apply to a tank which is mounted centrally in relation to the framework 10 (this variant is not shown).

As clearly shown in fig. 4, 5 and 6, plates 65 of a type which are in and for

known and adapted to register pressure variations and transfer them via channels, e.g. channel 66, to suitable instruments on board the maneuvering and steering pontoon. Variations in the water pressure which affect the plate 65" are thus registered. This

design example is of course only intended as an illustrative example and can be replaced by other devices with a similar function, namely to warn via variations in the water pressure affecting the side of the framework, that the framework, instead of sliding forward parallel to the pipe to be laid in the trench, is pulled in a deviant position and thus does not complete his task.

In the figures (especially fig. 6) an auxiliary device is also shown, so that the len is removed from the framework 10 for cutting algae in front of the dredger blades. This device comprises two half-frames 166, each of which comprises a boom 67 for connection with the framework 10 and a U-shaped cross bar 68, to which the front end of the boom 67 is attached. The ends of the crossbar 68 carry a plate 69, on which disc-shaped cutting means 70 are rotatably mounted.

Behind the crossbar 68, a rotatable roller 71 is mounted between brackets 72, which are attached to the boom 67 or to another boom 73.

From fig. 3, the operation of the cutting device will be easily understood. Here, the device moves forward in the direction indicated by arrow 46.

In fig. 8, 9 and 10, there is illustrated a device which is adapted to prevent the metal plate 74 by means of which the shape of the joints 75 of the pipe 11 which has already been laid out is built up, disturbs the sliding movement of the framework 10 along the pipe 11, in particular at the set of rollers 26.

Such disturbance possibly occurs because the metal plate 74, which is quite thin, is easily deformed and can open along a generatrix parallel to the tube 11. The metal plate can then get caught in the rollers 26 or in other parts of the framework and can easily cause damage on the pipe and/or the burial device.

In front of each set of rollers 26, a cantilevered, funnel-like device is mounted on the framework 10 for this purpose. It comprises two valves 77" which are articulated to each other along a generatrix by means of a hinge 78. The device 76 tapers towards the set of rollers 26.

The two valves 77 in reality enclose most of the circumference of the pipe 11 and cause the metal plate 74 to remain close or come closer and adhere to the surface of the pipe 11.

In practice, other embodiments of the device 76 can of course be used which are technically and functionally equivalent.

As for the discharge channels for the dredged material, as indicated at 48, it should be noted that they converge into a single discharge channel 47.

As shown in fig. 12, the outlet channel 47 is extended behind the frame 10. It can be supported by buoys 49 and empties the material directly over the already buried pipe 11 at a place where the pipe 11 has obtained its stable, final position in relation to the bottom. When there is a risk of so-called secondary pollution or in the case of very fine, dredged material, which easily remains suspended in the water, the outlet end of the pipe 47 is preferably enlarged, so that the speed of the dredged material to be emptied from this outlet is significantly reduced and the material can concentrated over the already laid pipe, so that this is covered again. Apart from the bends 49, the outlet pipe 47 can be guided so that it remains flush with the axis of the laid pipe. This can be done using a support and sliding structure, e.g. as generally implied by lk7•

In fig. 11 and 12 two operating states for the device according to the invention are illustrated. This applies to trenching in a relatively shallow bottom (fig. II) and trenching at a greater depth (fig. 12).

In the first case, the pontoon or vessel 50, (which carries with it all aids, such as compressors, winches, etc.) is anchored by means of an anchor or a weight 51» Displacement takes place in relation to the anchor or the weight, e.g., by that the anchor rope 52 is wound up using a winch 53"

The ditching device hangs behind the pontoon by means of the loose rope *>k and is held in operating position by means of a connecting rope 55" which is controlled by a winch 56.

By letting the device hang from the loose rope 5^, the unfortunate effects of phenomena such as wave motion and the like can be avoided.

The presence of the stabilizing and regulating nipples is also an important feature in this regard.

In the case shown in fig. 12, i.e. when the lay-out takes place in deep water, the vessel 50A is anchored both in the bow and aft by means of the weights 51A°g 51B. The rear front anchor acts as a brake on displacement, so that the operation of the dredging and digging buckets is smooth. In this case, the device is suspended in the loose rope 5^A, but the pulling takes place via the rope pair 16, which extends around the track disks 15 which are mounted on the weight 51A.

In this case, the winch 57 will induce displacement when winding the ropes 16, while the rope 59 which establishes a connection with the weight 51B is wound by the brake winch 58.

The rotation speeds of the winches 57 and 58 are of course set in a mutual relationship, as a function of the optimal speed of the trenching.

If the displacement speed of the device, e.g. must be 1.5 m/min., the winding of the ropes 16 with the help of the winch must take place at a speed of 3 running meters per minute, while the rope 59 must be unwound from the rear brake winch at a speed of 1.5 running meters per minute.

In other words, the operating speed should be the same as the unwinding speed of the rear anchor rope, while the front tow rope should be wound at twice the speed.

In the embodiments according to the invention as discussed above, reference is made to a special pneumatic pump. In special cases, depending on the type of pipe to be laid in the trench and on the laying depth of the pipe in the bottom, it is however possible to replace the above pump with another pump type. In such cases, the outlet channels for the digging buckets are connected to the pump's inlet port according to the technique in the area.

Another important feature of the device according to the present invention is that the framework 10, which is completely symmetrical with respect to the longitudinal plane through the pipe axis, can be regulated depending on the diameter of the pipe to be laid in the trench. It is in reality sufficient to arrange cross beams 18 where a length of the cross beam can be introduced or removed in the area between the side booms 17. The horizontal, rubber-coated rollers 26 are then either correspondingly adjustable or replaceable. It is of course also possible to provide the cross beams 18 with a central part which is formed by extendable and retractable members as a function of the pipe 11 diameter.

In a further modification of the present invention, instead of being provided with crumbling drawbars or ploughs, the digging shovels can be designed with shaped side extensions which have the same function as the aforementioned first set of drawbars 39, with the addition of a drainage or suction effect.

Such a modification can be particularly advantageous for sandy and poorly cohesive bottoms.

The side extensions can be pipes in the shape of a clarinet mouthpiece, possibly fitted with a cutting edge.

Alternatively, small digging shovels can be arranged instead of the crumbling drawbars. From a structural point of view, the buckets are independent of the actual dredger buckets. These auxiliary vanes can either be fixed or movable between one

•rest position and an operating position.

Finally, it should be noted that further modifications and variations of the device according to the invention, both in terms of the structure and the combination of different features, can be carried out within the scope of the invention.

Claims (22)

1. Device for laying pipes in underwater trenches, where the device generally comprises a framework, which can be placed across the pipe to be laid in the trench, digging devices mounted on the framework and adapted to work along both sides of the pipe which is already far on the bottom, at least at the framework, so that two trenches of determinable depth are formed, close to and parallel to the pipe itself, possible drawbars or plows, which are fixed or retractable and adapted for crumbling the bottom zone directly under the pipe, and a pump for removing the material excavated by the digging organs, characterized by the fact that the framework comprises at least two screeds for lateral protection and regulation of the maximum digging depth, which screeds are either continuous or interrupted and have adjustable height in relation to the digging or dredging organ and a first set of crumbling drawbars, mounted on vertical rods, driven by weight arms which are actuated by betting devices and movable between a rest position ng where the drawbars run essentially parallel and laterally offset in relation to the pipe, and an operating position where the drawbars are in engagement with the bottom zone immediately below the pipe to be laid in the trench, digging shovels which form the digging organs and are mounted in pairs symmetrically with respect to the framework and in a number of two or more shovels for each side of the framework, where each shovel has a digging edge at a level that is fixed and lower than the level of the digging edge of the shovels located closest to the front in the direction of pulling of the framework; means for regulating the distance between the two inner sides of the framework, the regulation depending on the diameter of the pipe to be trenchedJ a second set of crumbling drawbars arranged at least in the front part of the framework and projecting down to engage in the bottom in front of the dredging blades. 2. Device as stated in claim 1, characterized in that the mowing consists of lengths which are articulated with each other by means of hinges with horizontal axes or by means of universal couplings. 3. Device as stated in claim 1, characterized in that the coulters are connected to the cross beams for the framework via vertical or inclined struts, formed by ram units with extendable and retractable pistons, where the free ends of the pistons are rotatably connected to the coulters. h. Device as specified in claims 1-3, characterized in that organs which register the pressure from the surrounding water are mounted on the side of the screed. 5. Device as stated in claim k, characterized in that the devices consist of plates that sense the lateral pressure affecting the framework, where the plates are connected to detectors mounted on board the towing and steering pontoon. 6. Device as stated in claim 1, characterized in that a funnel-like device is arranged upstream of each set of rollers for guiding and shifting the framework in relation to the pipe to be laid in the trench, which opens along a generatrix and is hinged along it diametrically opposed generatrix, where the device tapers towards nearby roller sets. 7. Device as specified in claim 1, characterized in that probes are assigned to the first set of drawbars, that the probes can be adjusted and triggered by contact with the pipe to be laid in the trench, so that the crumbling drawbars are brought back to their rest position. 8. Device as stated in claim 1, characterized in that the drawbars in the first set are retractable backwards. 9. Device as stated in claim 1, characterized in that each digging bucket is connected to a pneumatic pump body. 10. Device as stated in claim 1, characterized in that the second set of drawbars is arranged in front of each dredge bucket, where the drawbars are placed at stepped levels. 11. Device as stated in claim 1, characterized in that at least on the front digging buckets, devices are mounted which generate vibrational movement. 12. Device as stated in claims 11 and 1, characterized in that the device which generates vibration movement is mounted on said second set of drawbars. 13" Device as stated in claims 11 and 12, characterized in that the dredger blades or the crumbling cores are connected to the framework via elastic connections. 14. Device as stated in claim 1, characterized in that there is an opening in each dredging bucket where water is fed to the inner cavity in the same bucket through said opening. 15" Device as stated in claim 14, characterized in that the opening is connected to a small pipe for water supply under adjustable pressure and adjustable flow rate, where the small pipe for each blade is connected to a common collection line. 16. Device as stated in claim 1, characterized in that the outlet channel from each bucket is connected to an outlet pump with an outlet pipe that extends behind the framework in relation to the direction of pull so that dredged material is emptied onto the pipe that has already been laid out, at a distance that is not less than that where the pipe has reached its final, stable position in relation to the bottom. 17. Device as stated in claim 16, characterized in that the outlet pipe is supported at a certain depth by means of buoys. 18. An arrangement.-/as stated in claims 16 and 17, characterized in that the outlet end of the outlet pipe is enlarged so that the speed of the emptied material is reduced. 19• Device as stated in claim 16, characterized in that the outlet pipe is supported at the outlet end by means of a support and sliding structure. 20. Device as stated in claim 1, characterized in that it comprises pairs of front and/or rear arms, which are articulated with the framework and are kept in contact with the surface of the pipe to be laid in the trench by sliding contact means, preferably rolls. 21. Device as stated in claim 1, characterized in that a tank is centrally mounted on the framework, and that the tank is either supplied with compressed air or with ballast water. 22. Device as specified in claim 1, characterized in that a device for cutting algae is provided with cutting discs and mounted in front of the framework.